Investigation on daily or every three days supplementation with protein or protein and starch of cattle fed tropical forage

Abstract

Effects of daily or every three days supplementation with protein or protein and starch on intake, digestion, rumen dynamics, urinary excretion characteristics, nitrogen (N) balance and, liver function of cattle fed high-quality tropical forage were evaluated. Five Nellore heifers (299 ± 7.5 kg of body weight) fitted with ruminal and abomasal cannulas were used in a 5 × 5 Latin square design. Treatments included: control; 200 g of crude protein (CP) daily; 200 g of CP and 400 g of starch daily; 600 g of CP every three days; 600 g of CP and 1,200 g of starch every three days. Supplements enhanced (P < 0.02) CP and digested organic matter intake. Among supplemented animals, every three days (infrequent) supplementation decreased (P < 0.05) forage intake. Supplements enhanced (P < 0.01) total organic matter and CP digestibility. Ruminal N balance was also enhanced (P < 0.01) by supplements, changing from negative in control to positive in supplemented animals. On average, supplements enhanced (P < 0.01) rumen ammonia nitrogen (RAN) compared to the control. However, an interaction (P < 0.01) between treatments and days pointed out that RAN levels were constant over days for control and frequently supplemented animals, but for infrequent supplemented animals it was greater on supplementation day and then decreased in the two following days. Supplements improved (P < 0.01) N balance, but without (P ≥ 0.43) differences among them. On average, supplemented animals presented greater (P < 0.01) urinary-N excretion (UNE) than control. Supplements, on average, also enhanced (P < 0.01) serum urea-N (SUN). Among supplemented animals, addition of starch to supplements reduced (P < 0.04) UNE, but it did not result in greater N retention. There was (P < 0.01) an interaction between days and treatments for SUN and the urinary excretion characteristics. Non-supplemented and daily supplemented animals presented constant values for these variables along the three-day-supplementation cycle. However, in animals supplemented every three days changes in SUN levels resulted in an untypical decoupling between SUN and RAN throughout the supplementation cycle. While RAN peaked on supplementation day, SUN peaked a day latter, which seem to be supported by a greater urea-N salvage in kidneys on day after supplementation. The changes in SUN and urinary excretion pointed out that the day after supplementation seem to be important for N salvage in ruminants infrequently supplemented. In summary, supplemented animals have greater N balance. Starch addition to protein supplements does not enhance N balance.